1. Field of the Invention
The present invention relates to radio receivers for direct sequence spread spectrum communications, and more particularly, to a radio system utilizing digital signal processing techniques that enhance receiver performance for operation in a multipath rich environment.
2. Description of Related Art
All radio systems are susceptible to a phenomenon known as multipath transmission. Ideally, a radio signal travels along a direct path between a transmitter and a receiver. In practice, however, the radio signal takes a less direct path resulting from reflection of the signal off of objects disposed between the transmitter and the receiver. Indoor radio systems within residential, office and industrial structures are particularly prone to multipath reflections due to the greater number of reflective surfaces disposed within the environment. The multipath transmissions tend to degrade overall performance of the radio system, since the desired signal becomes obscured by the numerous time-delayed versions of the original signal presented to the receiver. As a result, the operational range of the radio system is decreased.
The simplest technique for coping with multipath transmission is to isolate the strongest radio signal from the various multipath signals. The magnitude of a received signal from each path is a function of the distance traveled, the amount of signal power reflected off of each surface, and the relative antenna polarity. Generally, the radio receiver will lock onto the signal having the greatest magnitude, and treat the other multipath signals as noise which is simply filtered out by the radio receiver. This technique is easy to implement but not ideal because it tends to reduce the sensitivity of the radio system by increasing the threshold level for distinguishing noise, which reduces the effective operational range of the radio system.
More sophisticated radio systems utilize antenna diversity techniques for selecting between signals received at each of a plurality of antennas. The electric field levels received by each antenna are integrated over a pre-determined period of time to select the antenna with the highest time integrated value as the true signal source. Examples of radio systems utilizing antenna diversity are disclosed in: (a) U.S. Pat. No. 5,263,180 to Hirayama et al. for SPACE DIVERSITY RECEPTION SYSTEM; (b) U.S. Pat. No. 5,276,920 to Kuisma for ANTENNA SELECTION SWITCH FOR A DIVERSITY ANTENNA; and (c) U.S. Pat. No. 5,241,701 to Andoh for ANTENNA SELECTING DIVERSITY RECEIVING APPARATUS. While the selection between antennas improves performance of the radio system, the multipath signal is still present on the selected antenna and is treated as an interfering noise source. Thus, the receiver sensitivity must still be reduced to minimize the effects of the multipath signal which appears as noise, reducing the effective operational range of the radio system.
Spread spectrum modulation techniques are increasingly popular for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency (RF) carrier is modulated by a digital code sequence having a bit rate much higher than that of the information signal. These systems are known as "direct sequence" modulation systems. One example of a direct sequence spread spectrum system includes the RF carrier modulated by two data streams in quadrature with each one having one phase when the data stream code sequence represents a data "one" and 180.degree. phase shift when the data stream code sequence represents a data "zero." This type of modulation is commonly referred to as Quadrature Phase Shift Key (QPSK) modulation.
The direct sequence spread spectrum radio receiver samples the received signal several times over the transmission time of each individual data bit. The discrete samples, referred to as "bins," are correlated with the original digital code sequence to identify "hits" or matches that represent valid data. The number of hits for a particular bin signal is referred to herein as its magnitude. The bin having the greatest magnitude is selected by the receiver as being most likely to contain reliable and valid data.
Since the received signals over several bit times will vary as a result of movement of the radio and/or objects within the transmission path, the number of hits for the respective bins will also vary in an unpredictable manner. The radio receiver will lock onto a particular bin as providing valid data, however, if there is a sudden drop in hits for that bin number due to multipath reflection, the bit synchronization logic may attempt to lock onto a different bin. This may increase the chance of data corruption if the different bin does not contain valid or reliable data, or may cause the receiver to detect a false bit or miss a bit entirely depending on the time delay of the received signal. To minimize false synchronization on multipath signals, the tracking threshold is normally set to a high level to mask the lower magnitude signals. While this cuts down on false synchronization, it further compromises the ability of the radio system to lock onto signals having low power levels.
Thus, it would be desirable to provide a radio receiver for direct sequence spread spectrum communications that minimizes false synchronization due to multipath interference yet is sensitive enough to lock onto low power signals. A radio receiver that utilizes all received multipath signals to minimize false synchronization would allow the tracking threshold to be lowered to obtain greater overall system sensitivity.